Retail sales were on the menu this morning, as several companies posted their January same-store sales results. Retailers were mostly beating expectations, but the market quickly turned south on the sector as trading progressed through this morning. What we're seeing here is an interesting situation where January 2013 contained an extra fifth selling week vs. January 2012. Several stocks seemed to gap higher at the open based on the headline comps (which mostly included the extra week), but then sold off as investors filtered out the results to account for the extra week.

Kohl's (NYSE:KSS), for instance, posted a +13.3% January comp result and the stock opened approximately 3% higher. But when one reads further and finds that the extra week included $169 million of sales (14.9% of total sales for the month), it's not nearly as impressive; the stock is trading negative now. It was a similar story for Limited Brands (LTD), where a +9% comparable sales result included approximately $125 million from the extra week. That stock is down 3%.

One company that still managed to impress, though, was Macy's (NYSE:M), which posted a +34.6% comp that was still quite good at +11.7% after subtracting the extra week. Management noted: "Simply put, January was an outstanding month for Macy's." Gap (NYSE:GPS) posted a +8% comp for January, besting the +4% consensus. However, the press release didn't specify about the extra week. That stock opened slightly higher, but the Street has turned sharply negative on it, down 4%.

In all, the retail sector is underperforming the market on the day as sales, on balance, just weren't all that impressive compared to last year. At the same time, how can you blame retail sales for not being super great, considering the average consumer must have experienced a bit of paycheck shock at the start of the new year with that 2% payroll tax increase?